How to connect the Raspberry Pi to a Bluetooth heart rate monitor.

$ wget http://www.kernel.org/pub/linux/bluetooth/bluez-5.37.tar.xz
$ tar -xvf bluez-5.37.tar
$ ./configure --disable-systemd
$ make
$ sudo make install

$ sudo hciconfig hci0 up
$ sudo hcitool -i hci0 lescan

LE Scan...
00:D2:D0:94:C2:C0 (unknown)
00:D2:D0:94:C2:C0 Polar H7 97C4C011

$ sudo hciconfig hci0 down
$ sudo service bluetooth stop

d, err := gatt.NewDevice(option.DefaultClientOptions...)
if err != nil {
	log.Printf("ERROR: Unable to get bluetooth device.")
	return
}

// Register handlers.
d.Handle(
	gatt.PeripheralDiscovered(func(p gatt.Peripheral, a *gatt.Advertisement, rssi int) {
		onPeriphDiscovered(p, a, rssi, deviceID)
	}),
	gatt.PeripheralConnected(func(p gatt.Peripheral, err error) {
		onPeriphConnected(p, done, err)
	}),
	gatt.PeripheralDisconnected(func(p gatt.Peripheral, err error) {
		onPeriphDisconnected(p, done, err)
	}),
)

d.Init(onStateChanged)

// Get the heart rate service which is identified by the UUID: \x180d
ss, err := p.DiscoverServices([]gatt.UUID{gatt.MustParseUUID("180d")})
if err != nil {
	log.Printf("ERROR: Failed to discover services - %s\n", err)
	return
}

for _, s := range ss {
	// Get the heart rate measurement characteristic which is identified by the UUID: \x2a37
	cs, err := p.DiscoverCharacteristics([]gatt.UUID{gatt.MustParseUUID("2a37")}, s)
	if err != nil {
		log.Printf("ERROR: Failed to discover characteristics - %s\n", err)
		continue
	}

	for _, c := range cs {
		// Read the characteristic.
		if (c.Properties() & gatt.CharRead) != 0 {
			_, err := p.ReadCharacteristic(c)
			if err != nil {
				log.Printf("ERROR: Failed to read characteristic - %s\n", err)
				continue
			}
		}

		// Discover the characteristic descriptors.
		_, err := p.DiscoverDescriptors(nil, c)
		if err != nil {
			log.Printf("ERROR: Failed to discover descriptors - %s\n", err)
			continue
		}

		// Subscribe to any notifications from the characteristic.
		if (c.Properties() & (gatt.CharNotify | gatt.CharIndicate)) != 0 {

			err := p.SetNotifyValue(c, func(c *gatt.Characteristic, b []byte, err error) {
				heartRate := binary.LittleEndian.Uint16(append([]byte(b[1:2]), []byte{0}...))
				contact := binary.LittleEndian.Uint16(append([]byte(b[0:1]), []byte{0, 0}...))

				// Notify if the HRM has skin contact, and the current measured Heart rate.
				if contact == 6 || contact == 22 {
					fmt.Printf("1,%d\n", heartRate)
				} else {
					fmt.Printf("0,%d\n", heartRate)
				}
			})

			if err != nil {
				log.Printf("ERROR: Failed to subscribe characteristic - %s\n", err)
				continue
			}
		}

	}
	log.Println()
}